1. Field
The present invention is concerned with novel 4-cyano-3-fluorophenyl esters, their manufacture, liquid crystalline mixtures which contain these compounds as well as their use for electro-optical purposes.
2. Discussion
Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a "twisted nematic" structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
Liquid crystals must have a good chemical and thermal stability and a good stability towards electric fields and electro-magnetic radiation. Moreover, liquid crystals should have a low viscosity and in the cells should give short response times, a low threshold potential and a high contrast. They should have a suitable mesophase, for example a nematic or cholesteric mesophase for the aforementioned cells. Other properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the type of cell.
Liquid crystals are generally used as mixtures, because the properties which are desired for the respective application can be optimized by mixing different components. It is therefore important that the components have a good miscibility with one another.